1. Technical Field
The present invention relates in general to an improved data processing system and in particular to an improved data storage subsystem for use with a data processing system. Still more particularly, the present invention relates to an improved method and system for maintaining data accuracy for data stored within a direct access storage device.
2. Description of the Related Art
Modern computer systems often use a plurality of storage devices or memories in order to store data associated with that computer. Many such storage devices are categorized as so-called "direct access storage devices" (DASD) in that the access time for data within such a device is effectively independent of the location of the data. One example of such a storage device is a magnetic disk drive memory.
In systems which utilize such memory devices, accuracy in maintenance of the data stored therein is of particular importance. The writing and reading of data to such devices carries with it an associated probability of error. Procedures and devices have been developed which can detect and, in some cases, correct the errors which occur during the reading and writing of data to such disks. For example, with respect to a commonly utilized disk drive memory system, the disk is divided into a plurality of sectors, each sector having the same, predetermined size. Each sector has a particular header field which gives the sector a unique address, a header field code which allows for the detection of errors in the header field, a data field of variable length and, so-called Error Correction Codes (ECC) which allow for the detection and correction of errors within the data.
Thus, when a magnetic storage disk has been written to, the disk controller reads the header field and header field code. If the sector designated is the desired sector and no header field error is detected, the new data for that sector is written into the data field and new data ECC is calculated and written into the ECC field.
While such systems function admirably, there occur situations periodically where the magnetic storage media for a direct access storage device requires remedial action due to errors attributable to the physical state of the media. When such remedial action has been complete, a typical result is that more data is lost than was originally damaged or affected within a defective track sector, block or other addressable memory segment. Thus, while such systems are highly efficient in detecting errors within individual addressable portions of a magnetic storage media the loss of data within that section may be unnecessarily large.
Alternately, upon completion of the remedial action, a subsequent read of the contents of that addressable memory segment may result in the transfer of some data content with no error indication, even though the data actually transferred is unrelated to the data which was present prior to the remedial action.
Thus, is should be apparent that a need exists for an Error Correction Code (ECC) system which permits the data within a track sector, block or segment to be maintained separate from the physical media within which the data is stored.